Studies with a succinylated derivative of concanavalin A (S-Con A ) indicate that Con A binding surface antigens of Entamoeba histolytica are involved in trophozoite attachment. Inhibition of amebic attachment by S-Con A was not due to agglutination or capping and was not produced by anti-amebic sera or other lesions. Giardia lamblia and E. histolytica trophozoites were killed by thiol blocking reagents which remain outside of cells. Killing was prevented by cysteine or by bovine serum albumin. This is evidence for crucial thiol groups on the parasites' surface. Our new, precise quantitative method of determining viability of G. lamblia and E. histolytica trophozoites in vitro was utilized in drug studies. The susceptibilities of these unrelated intestinal pathogens to a variety of drugs were similar. However, G. lamblia was more sensitive to quinacrine and less sensitive to several inhibitors of protein synthesis. A new strain of G. lamblia has been isolated from a patient who had been treated unsuccessfully with quinacrine and with metronidazole. In vitro, the organism was sensitive to both drugs.